Field of the Invention
The present invention relates to a sheet detecting apparatus which detects a conveyed sheet and an image forming apparatus and an image reading apparatus which include the same.
Description of the Related Art
In general, a sheet conveyor of an image forming apparatus is equipped with a sheet conveying apparatus which conveys a sheet to an image forming portion or a discharge tray. The sheet conveying apparatus is equipped with a sensor which detects a sheet in order to control a sheet conveying speed or detect a jam (for example, see U.S. Pat. No. 6,011,948).
A sheet detecting apparatus 620 as a comparative example is illustrated in FIGS. 14 to 15C. As illustrated in FIG. 14, the sheet detecting apparatus 620 of the comparative example is provided at the downstream side in the sheet conveying direction of a pair of conveying rollers 618 and 619 closest to a transfer position where an image formed in an image forming portion is transferred. The sheet detecting apparatus 620 includes an abutting portion 623 which abuts against a sheet S, a photo sensor 624, a light shielding portion 625 which shields an optical path from a light emitting portion to a light receiving portion of the photo sensor 624, and a stopper 626 which positions the abutting portion 623 at a home position.
The abutting portion 623 is provided so as to be rotatable about a rotation shaft 627. The abutting portion 623 is formed so as to return to a home position H illustrated in FIG. 15C by the pressure of a twist coil spring 628. The light shielding portion 625 is integrally formed with the abutting portion 623, and rotates about the rotation shaft 627 along with the abutting portion 623.
As illustrated in FIG. 15A, when a leading end of the sheet S abuts against the abutting portion 623, the abutting portion 623 rotates about the rotation shaft 627 from the home position H illustrated in FIG. 15C in the direction of the arrow a of FIG. 15A, and hence the light shielding portion 625 shields the optical path of the photo sensor 624. When the photo sensor 624 detects a state where the optical path is shielded, the sheet detecting apparatus 620 recognizes a state where the leading end of the sheet S reaches the abutting portion 623.
Subsequently, the sheet S is conveyed while contacting a front end of the abutting portion 623. As illustrated in FIG. 15B, when a tail end of the sheet S passes by the abutting portion 623, the abutting portion 623 rotates in the direction of the arrow b illustrated in FIG. 15C by the biasing force of the twist coil spring 628 so as to return to the home position H. At this time, the light shielding portion 625 is retracted from the optical path of the photo sensor 624, and the light receiving portion of the photo sensor 624 receives the light emitted from the light emitting portion again. Accordingly, the sheet detecting apparatus 620 recognizes a state where the tail end of the sheet S passes by the abutting portion 623.
In recent years, there has been a demand for improving the throughput (the processing capacity per unit time) in the image forming apparatus. There is a case where a gap (hereinafter, referred to as a “sheet gap”) from the tail end of the precedent sheet S to the leading end of the subsequent sheet S is shortened in order to improve the throughput in the image forming apparatus. In this case, the sheet detecting apparatus 620 needs to handle the short sheet gap.
The abutting portion 623 of the comparative example rotates while being pressed by the sheet S when the leading end of the sheet S passing through the pair of conveying rollers 618 and 619 abuts against the abutting portion 623. Then, when the tail end of the sheet S is separated from the abutting portion 623, the abutting portion 623 returns to the home position H while being biased by the twist coil spring 628 so that the abutting portion 623 reversely rotates. For that reason, as illustrated in FIG. 15B, the distance necessary as the sheet gap D is the sum of a distance D2 and a mechanical loss amount D1 as a temporal loss amount caused by mechanical operation described in the following Equation 1.D=D1+D2   [Equation 1]
As illustrated in FIG. 15B, the mechanical loss amount D1 is the following distance. That is, the distance corresponds to a distance in which the abutting portion 623 rotates from the position where the tail end of the precedent sheet S passes by the abutting portion 623 about the rotation shaft 627 by the biasing force of the twist coil spring 628 and moves to the home position H illustrated in FIG. 15C.
Meanwhile, the distance D2 is as below. Here, the time until the abutting portion 623 moves by the mechanical loss amount D1 in a manner such that the abutting portion 623 returns to the home position H as illustrated in FIG. 15C after the tail end of the sheet S is separated from the abutting portion 623 as illustrated in FIG. 15B is indicated by Δt. Then, the distance D2 becomes a distance obtained by multiplying the conveying speed V of the sheet S conveyed while being nipped by the pair of conveying rollers 618 and 619 by Δt as illustrated in the following Equation 2.D2=Δt×V   [Equation 2]
Then, since Δt is shortened when the mechanical loss amount D1 is shortened, the distance D2 is also shortened depending on the mechanical loss amount D1 from Equation 2. Accordingly, the sheet gap D is shortened depending on the mechanical loss amount D1 from the above-described Equation 1. From the description above, there is a need to shorten the mechanical loss amount D1 in order to shorten the sheet gap D between the precedent sheet S and the subsequent sheet S.
Here, there are proposed techniques in Japanese Patent Laid-Open No. 2008-001465 and U.S. Patent Application Publication No. 2012/181,741 A1. In Japanese Patent Laid-Open No. 2008-001465, a mechanical loss amount may be shortened by inclining a rotation shaft of a sensor flag with respect to a sheet conveying direction h when seen from a direction of a normal line i of a surface of a sheet S. In this way, when the rotation shaft of the sensor flag is obliquely inclined, the falling amount of the sensor in the sheet conveying direction h at the time in which the sensor becomes an ON state due to the passage of the sheet is smaller than that of the comparative example, and hence the mechanical loss amount may be decreased.
Further, in U.S. Patent Application Publication No. 2012/181,741 A1, a sensor flag is not formed in a swing type as in the comparative example, and the sensor flag rotates by one revolution whenever each sheet S passes by the sensor flag. In this way, the mechanical loss amount is decreased.
However, in the configuration of Japanese Patent Laid-Open No. 2008-001465, for example, as illustrated in FIG. 9C, the rotation shaft of the sensor flag is disposed so as to be inclined by 45° as an actual angle with respect to the sheet conveying direction h when seen from the direction of the normal line i of the surface of the sheet S. In that case, the mechanical loss amount is improved only by about 30% compared to the comparative example. Further, in U.S. Patent Application Publication No. 2012/181,741 A1, since almost ten components are used, a space is needed in the sheet conveying direction h.
It is desirable to provide a sheet detecting apparatus having a small mechanical loss amount by simplifying a configuration and saving a space.